Semiconducting composition



Oct. 1, 1946.

D. E. EDGAR arm. 2,408,416 SEMI-C(JNDUCTING COMPOSITION Filed Feb. 18,1944 5 Jami condwctz'zg'coafing' Dona/,Zd E.Ed ar David Sal vanINVHVTORJ W (9 AYQTORW Patented Oct. 1, 1946 SEMICONDUCTING COMPOSITIONDonald E. Edgar,

Westport, and David Sullivan, Fairfleid,onn., minors to E. I.

Dei., a corporation of Delaware du Pont de Nemours 8: Company,Wilmington,

Application February 18, 1944, Serial No. 522,979

This invention relates to a semi-conducting coating composition,particularly adapted for .coating insulated high tension electric cablesand more particularly to a semi-conducting tape which maintains thevoltage gradient at the surface of the insulation below a value abovewhich harmful electrical disturbances occur.

Heretofore it has been common to dissipate high voltage charges whichsurround electric cables carrying very high voltages, that is of theorder of 2,000 volts or more, by means of a metallic conducting shieldover an insulating medium which may be in the form of oil saturatedpaper in combination with rubber, braid, regenerated cellulose and thelike. In ordinary use it is necessary to dissipate or dischargethecharges which build up on such a cable and this has been done byvarious means, such as surrounding the cable with the thin metallicsheath mentioned above. Metals, however, have the disadvantage thatfrequent bendings of the cable tend to develop air gaps because of theseparation of the metal shield from the adjacent insulation. When suchgaps are formed, arcing occurs which eventually causes a failure of theinsulating medium. It has also been proposed to provide a shield whichis more flexible than metal. Such shields have been prepared by aconducting paint, composition of rubber, or other film-forming vehiclein combination with metallic particles. None of these expedients' havebeen entirely satisfac tory due primarily to the failure of thefilmforming vehicle when immersed in a liquid tures, thus protecting theinsulation used in electric cable construction from injury anddeteriorathan by exposure to such materials. Another object is therovision of a semi-conducting coating which is tough and elastic andwhich will retain its flexibility and remain intact over a wide range oftemperatures without deterioration. A further and more particular objectis the pro- 15 Claims. (Cl. 114-102) vision of a semi-conducting coatingwhich is highly resistant to mineral oils and greases, even when exposedto such materials at elevated temperatures. Another object is theprovision of a semi-conducting composition which does not contaminateinsulating oils with decomposition products formed during normal servicewith resultant failure of insulating properties such as is commonlyencountered with previously employed materials, for example, rubber. Afurther object is the provision of an improved semi-conducting tape,particularly adapted for use in the manufacture of high tension electriccables. A still further object is the provision or an insulated hightension electric cable in which the accumulation of static charge issubstantially eliminated,

thereby avoiding deteriorating and breakdown effects in the insulatingcovering over the cable. Other objects will appear as the description ofthe invention proceeds.

These objects are accomplished in accordance with the present inventionby means of a semiconducting coating composition comprising finelydivided carbon black of low electrical resistivity, preferably not overabout 1.0 ohm per 1" cube dispersed in a vehicle containing an alkydresin as the essential film-forming ingredient. In the preferredembodiment a semi-conducting composition comprising an alkyd resin andacetylene black described hereinafter is applied to a Iabric tape whichis adapted for covering the organic insulation commonly employed inelectric cable construction.

In the drawing Figures 1, 2, and 3 represent diagrammatic partialsections of a high tension cable prepared according to the presentinvention. Figure 4 is a section of fabric coated with a semiconductingcomposition described in the examples. In Figure 1, the conductor isshown as l which may be a solid conductor or a number ol' copper wiresas shown. Next to the conductor is a layer of oiled paper shown as 2.Over this is applied a semi-conductinglayer applied as a tape made up ofa semi-conducting coating 5 on a sup ort which in Figure 1 is shown aspaper. Figure 2 is a modification of Figure l in which the oiled paperis replaced by rubber insulation 4. A semi-conducting cloth layer 1having a coating 5 is applied over the rubber insulation. The layers 5and! are made up from a spirally wound tape over the rubber, the tape orcloth 1 being coated or impregnated with a semi-conducting composition 5described in the examples. The braid 6 is provided to protect thesemi-conducting layer 5. Figure 3 is a modification in which the copperconductor i is wound directly with a tape made up of a cloth base 1 anda semi-conducting composition 5. Over this tape is applied a layer ofrubber insulation 8.

In Figure 4 a section of the semi-conducting tape is shown having afabric base 1 and a composition 5 which is more fully described below.

The following examples are given by way of illustration only and nolimitations are intended thereby except as indicated in the appendedclaims.

Example 1 Per cent by weight Alkyd resin solution 43.8 Polyvinyl butyralresin (Butacite) 9.8 Urea-formaldehyde resin solution 7.3 Dlcaprylphthalate 4.9 Acetylene black 34.2

The alkyd resin solution of this example consisted of a 60% solution ofa 53% castor oil modifled diglycerol-triphthalate resin in a volatilesolvent vehicle consisting of Per cent by weight Isobutyl alcohol 80Petroleum naphthas (B. P. 135-2l5 C.) 20

The polyvinyl butyral-resin may be prepared by any of the methods wellknown in the art, for example, by treating polyvinyl acetate with ahydrolyzing agent such as sulfuric acid, followed by condensation withbutyraldehyde to give a solid, fusible resin soluble in certain organicsolvents. The resin of th example had a hydroxyl content of about Theviscosity of the resin was '75 centipoises determined on a 5% solutionof the resin in 23 denaturedalcohol.

The urea-formaldehyde resin solution consisted of a 60% solution inbutyl alcohol of a butyl alcohol modified urea-formaldehyde resin asdescribed in Edgar et a1. U. S. Patent 2,191,957.

The acetylene black which imparts semi-conducting properties to thecomposition may be prepared in accordance with the description containedin Canadian Chemistry and Metallurgy May 1933, pages 93-95. In additionto this carbon black, there are now on the market many blacks with highconducting properties. Chapter 7, page 67 of the book ColumbianColloidal Carbons, published in 1938 by the Columbian Carbon Company,defines a suitable type of black as having an electrical resistance inohms per 1" cube of 0.152 to 0.217, although a somewhat higherresistance up to about 1.0 ohm per 1" cube is satisfactory. In theclaims such blacks are termed low resistance carbon blacks.

The composition was prepared by combining the Butacite resin anddicapryl phthalate on differential speed conventional rubber roller millafter Which the acetylene black, alkyd resin and urea-formaldehyde resinare incorporated, preferably by adding alternate portions of theacetylene black and resin solutions to the material already being workedon the rolls. The materials are milled for about minutes or until ahomogeneous mixture is obtained. This step in the preparation of thecomposition may also be carried out in a kneading machine, preferably ofthe heavy duty type such as a Banbury mixer. The resulting plastic stockfrom the roller mill 'was cut into small pieces and churned with xylolin the proportion of about 1 part of plastic to 0.4 part of xylol in anagitator mixer until a homogeneous solution is obtained. If the plasticis prepared in a kneading machine the solvent may be added direct withcontinued kneading.

The composition thus reduced to spreader viscosity with xylol wasapplied to'a fabric base consisting of cotton sheeting weighing about5.35

. yardsper pound by a conventional knife spreader coating machine.Usually three coats sufiice to obtain a desired thickness of about 3mils, although a fewer or greater number of coats may be applied ifpreferred. Each coat was dried at 240 F. for approximately 2 minutes inorder to remove the volatile solvent and to promote the reaction betweenthe Butacite resin and the insolubillzing agent, in this instance theurea- -formaldehyde resin. When a cloth base is used, it is coated withabout 2 to 4 ounces per square yard of the semi-conducting composition.With a suitable paper base which may be used alternatively as a support,about 1 ounce per square yard usually sufilces.

The electrical resistance of the tape or an unsupported film of thesemi-conducting composition of Example 1 was 300 ohms per inch square.

Samples of fabric tape coated with the composition of Example 1 wereimmersed in mineral oil of the type commonly employed in electricaltransformers at a temperature of 180 C. After exposur under theseconditions for a period of 6 months the coating was still intact and noevidence of deterioration was observed whereas tapes coated withcompositions based on rubber or neoprene and exposed to the sameconditions failed in less than 48 hours and after 15 days, respectively,as shown by development of porosity in the coating which allows thepenetration of the mineral oil and subsequent electrical failure of theinsulation surrounding the conductor. These The alkyd resin solution andthe polyvinyl butyral resin used in this composition were of the sametype as described in Example 1.

The insolubilizing agent for the polyvinyl resin consisted of a 70%solution in butanol of dibutyl ether of dimethylol urea.

The Spheron N carbon black used in this example consisted of a specialchannel carbon black marketed by Godfrey L Cabot, -Inc., which is alsocharacterized by low electrical resistivity.

The composition was prepared in accordance with the procedure of Example1 and results similar to those obtained in Example 1 were also obtainedin this instance.

The electrical resistance of the treated tape or unsupported film of thesemi-conducting composition is usually between about and 500 ohms perinch square although in some instances the tape may have a resistance ofabout 1,000 ohms For present purposes the oil length of the modifiedalkyd resin is ordinarily between 30 and 60%. Drying oils are notdesired because of their greater tendency tooxidize and causeembrittling in the present utility where heat and the presence of ozonepromote such tendencies. Phthalic anhydride is the preferred acidradical in the alkyd resin ingredient of the improved compositions butother carboxylic acids such as succinic, malic, adipic, maleic, sebacic,etc., may be substituted wholly or in part for the phthalic anhydride,but best results may be secured when phthalic anhydride is employed asthe sole acid radical or at least in preponderant' amounts. Suitablealkyd resins for the new compositions may also be prepared from otherpolyhydrie alcohols including ethylene glycol andhigher homologs,diethylene glycol and other polyglycols, poly lycerols, pentaerythritoland sorbitol.

Although the use of polyvinyl butyral is shown in the examples, theinvention is also applicable with other polyvinyl acetal resinsincluding reaction products of hydrolyzed (or partially hydrolyzed)polyvinyl esters with the lower alkyl aldehydes such as formaldehyde,acetaldehycle, propionaldehyde and valeraldehyde. The polyvinyl acetaltoughens and strengthens the coating and facilitates application byspreading or calendering to fabrics and other sheet material andfunctions also to prevent excessive penetration into such porous bases.

The butanol modified urea-formaldehyde resin shown in the Example 1reacts with the polyvinyl acetals as a cross-linking agent for thedouble bonds resulting in an insolubilizing of the acetal and areduction in plastic flow. Other agents which function in a similarmanner and may be used in place of the butanol modifiedurea-formaldehyde condensation product shown in the Example 1 includeN,N'-bis(alkoxymethyl) urons, described in co-pending application S. N.438,949 to Maxwell, filed April 14, 1942, now Patent No. 2,373,135,dated April 10, 1945, alcohol modified melamine-formaldehydecondensation products of co-pending application S. N.'491,400 to Vaala,filed June 18, 1943 (also U. S. Patent 2,197,357), dimethylol urea andalkyl ethers of dimethylol urea as described in co-pending applicationS. N. 489,805 to Vaala, filed June 5, 1943, and organic complexes of theWerner type in which a trivalent chromium atom is coordinated with acarboxylic acido group having at least carbon atoms such as stearatechromic acetate as disclosed in co-pending application S. N. 498,981 toJohnson, filed August 17, 1943.

The plasticizers or softeners which may be used in the present inventioninclude conventional materials such as castor oil, blown castor oil,aryl' sulfonamides, aryl and alkyl esters of phosphoric acid, dibutyltartrate, dicyclohexyl phthalate, diethyl phthalate and the phthalateesters of the ether alcohols such as the butyl ester of ethylene glycolmonomethyl ether.

In preparing the new compositions from the alternative materialssuggested above, care should be taken to select those combinationswhich'will afford maximum resistance to penetration of mineral oil suchas is achieved with the specific compositions shown in the examples.

As previously indicated, the carbon black used in the presentcomposition has low electrical resistivity of between about 0.152 ohmper 1" cube and preferably not over 1.0 ohm per 1" cube. Acetylene black(Shawinigan Black) is of particular merit in the present compositions,al-

6 though Graphon, a graphitized carbon black, prepared from natural gasand treated in accordance with U. S. Patent 2,134,950 is alsorepresentative of suitable semi-conducting carbon blacks. Other specialforms of channel carbon black sold by Godfrey L. Cabot, Inc., under thetrade names Spheron N, "Spheron T and "Spheron C which have lowelectrical resistivities have also been found suitable for the presentpurposes.

These semi-conducting materials may be usedin the proportion of 1 partby weight of the carbon black to between about 0.25 and 4 parts byweight of vehicle solids including the resin components and theplasticizer. A ratio of vehicle solids of between about 1 to 2 parts to1 part of carbon black of the type defined above is preferred. If thereis any substantial departure from the limits indicated, the primaryobjectives of the invention are not attained. If the semi-conductingcarbon black is present in proportions substantially below the limitsestablished, the desired conductance is not obtained probably due toabsence of direct contact between particles and if the carbon black isincorporated in substantially greater proportions than the upper limitsuggested. there-are deficiencies in film properties of the coating.

' Another factor which has a contributing influence on the conductanceof the coating is the degree of dispersion of the semi-conducting carbonblack as controlled by the grinding conditions, particularly thegrinding cycle employed in dispersing the pigment in the vehicle.Extended grinding cycles apparently cause the particles of the carbonblack to become surrounded with the vehicle which is relativelyinsulating in character, thus adversely affecting the desiredconductance in the final coating.

As previously indicated, the coating may be used as a free film orcoated from solution directly on to the surface of the organicinsulation surrounding the conductor or alternatively and preferably thecoating is applied to one or both surfaces of a suitable supporting basesuch as fabrics of cloth, paper, asbestos'or fabric made from glassfibers and the coated support then wound spirally about the organicinsulation. A suitable paper support may consist of a conventionalinsulating paper of between about .003 and .0035 inch in thickness.Satisfactory cloth fabric support may consist of cotton sheetingweighing about 5.35 yards per pound.

The new compositions are of general utility for purposes where asemi-conducting surface is desired. However, a specific and importantuse resides in the construction of high tension electric cables employedin automotive ignition systems where the coating functions as a shieldto reduce or prevent the formation of static fields about the conductorwhich sometimes reach glow discharge or corona intensity with formationof excessive ozone and resultant rapid breakdown or failure in theelectrical insulation.

The invention is characterized by a number of important advantages. Inthe new compositions satisfactory electrical conductance is combinedwith exceptionally high resistance to hot mineral oils and greases overextended periods of time. It is in this latter respect that previouslyavailable similar coatings are particularly deficient. Not only are thenew compositions substantially impervious to hot oils and greases butthey are also quite resistant to water, ozone and acids such as areusually encountered during service in automotive ignition systems sothat the electrical conductance is not impaired by exposure to suchdeteriorating eflects. The products of the invention are iurtherdistinguished from inferior compositions previously available in thatthere is substantially no contamination of the insulating oils in thetreated paper insulation from decomposition products so that earlyfailure of the insulation surrounding the electric conduction isavoided. The improved coatings are tough and flexible at low and hightemperatures and suificiently elastic to conform to any curvatureimparted to the le without breaking. High tension electric cablesconstructed with the new compositions as a part thereof have anexceptionally long life and maintain uniform electrical conditions muchlonger than any heretofore in commercial use.

It is apparent that many widely different embodiments of the inventionmay be made without departing from the spirit and scope thereof and,therefore, it is not intended to be limited except as indicated in theappended claims.

We claim:

1. A semi-conducting coating composition particularly adapted for themanufacture of high tension electric cables comprising a resinousvehicle containing a non-drying oil modified alkyd resin, a polyvinylacetal resin, a ureaformaldehyde resin and a carbon black havingelectrical resistivity of less than about 1.0 ohm per 1" cube in theproportion of between about 0.25 and 4 parts by weight of said resinousvehicle to 1 part of said carbon black.

2. Composition of claim 1 in which the alkyd resin is a castor oilmodified diglyncerol triphthalate resin.

3. Composition of claim 1 in which the alkyd resin is a 30-60% castoroil modified digiycerol triphthalate resin.

4. Composition of claim 1 in which the carbon black has an electricalresistivity of between about 0.152 and 0.217 ohm per 1" cube.

5. Composition or claim 1 in which the carbon black is acetylene black.

6. Composition of claim 1 in which the carbon black is graphitizedcarbon black.

7. Composition of claim 1 in which the polyvinyl acetal resin ispolyvinyl butyral.

8. Composition of claim 1 in which the insolubilizing agent is a butanolmodified urea-formaldehyde resin.

9. An electric cable comprising an insulated conductor having an outerlayer of a composition containing the composition of claim 1. 7

10. A semi-conducting composition particularly adapted for themanufacture of high tension electric cables having approximately thefollowing composition:

Per cent by weight 53 castor oil modified alkyd resin solution solids)48$ Polyvinyl butyral resin 9.8 Butanol modified urea-formaldehyde resinsolution (60% solids) 7.3 Dicapryi phthalate 4.9 Acetylene black.. 34.2

11. A new composition of matter adapted to withsd adverse conditions ofmoisture. temperature and mineral oil comprising carbon black having anelectrical resistivity of less than 1 ohm per 1 inch cube, a non-dryingoil modl fied alkyd resin, a polyvinyl acetal resin, and a lowermonohydric aliphatic alcohol modified urea-formaldehyde resin, theelectrical resistance or the said composition being between about and500 ohms per inch square.

12. A semi-conducting tape particularly adapted for the manufacture ofhigh tension electric cables comprising a fabric base support carrying acoating containing a non-drying oil modified alkyd resin, a polyvinylacetal resin, a urea-formaldehyde resin and a carbon black having anelectrical resistivity of less than about 1.0 ohm per 1 inch cube in theproportion of between about 0.25 and 4 parts by weight of the resinousvehicle to 1 part of the said carbon black.

13. Product of claim 12 in which the tape consists of a fabric base.

14. Product of claim 12 in which the tape is paper.

15. The article of claim 12 in which the carbon black has a resistivityof between about 0.152 and 0.217 ohm per 1 inch cube.

DON E. EDGAR.- DAVID .1. SAN.

